asn't a PC of similar specs and size a hell of a lot cheaper still?
I think it is very difficult to compare Mac's and PC's. Looking at raw specs is very misleading. Ideally you should try the model you want to see whether it meets your needs.
I prefer to use my 867Mhz Mac rather than my 3Ghz PC (whether running Windows or Linux). The Mac seems at least as fast, if not faster, than the PC for all normal tasks (i.e. except for playing the latest games), including development. Given that, the Mac mini would be just fine (yes I want one too!)